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ABOUT THIS IMAGE:

This rich galaxy cluster, catalogued as Cl 0024+17, is allowing astronomers to probe the
distribution of dark matter in space. The blue streaks near the center of the image are the
smeared images of very distant galaxies that are not part of the cluster. The distant
galaxies appear distorted because their light is being bent and magnified by the powerful
gravity of Cl 0024+17, an effect called gravitational lensing.

Dark matter cannot be seen because it does not shine or reflect light. Astronomers can
only detect its influence by how its gravity affects light. By mapping the distorted light
created by gravitational lensing, astronomers can trace how dark matter is distributed in
the cluster. While mapping the dark matter, astronomers found a dark-matter ring near
the cluster's center. The ring's discovery is among the strongest evidence that dark matter
exists.

The Hubble observations were taken in November 2004 by the Advanced Camera for
Surveys.